And the Lord shall become King over all the earth; on that day shall
the Lord be one, and His name one.(Zech. 14:9)
Rashi's commentary reads,
*"shall the Lord be one: For all the nations shall abandon their
vanities and acknowledge Him, that He is one, and [that] no strange
deity is with Him,and His name one: That His name shall be mentioned by everyone.
Keil & Delitzsch's commentary reads,
"To this blessing there is added the higher spiritual blessing, that
Jehovah will be King over all the land, and His name alone will be
mentioned and revered.
Interestingly, Rashi's commentary includes the whole earth, whereas Keil & Delitzsch's only include Israel. They don't go into detail as to why; my suspicion is they are affected by a theological bias that prevents them from acknowledging the obvious.
I'm not a Hebrew scholar, but it appears the context is not one of the 'personages' of God, but that there is no more 'contest' as to who God is. There is but one deity, and that is God. That means any 'created being' cannot replace the pre-existent God.
The question of course is "What about Jesus?". and the answer is found in John 1:1-3,
" 1In the beginning was the Word, and the Word was with God, and the
Word was God. 2The same was in the beginning with God. 3All things
were made by him; and without him was not any thing made that was made.
God is one, there have been many rivals to Him. Zech. 13:2 says,
And it shall come to pass in that day, saith the Lord of hosts, that I
will cut off the names of the idols out of the land, and they shall no
more be remembered: and also I will cause the prophets and the unclean
spirit to pass out of the land.
The Lord alone and His name will be mentioned, no other idol will be mentioned. This is the context of Zech. 14:9.